I used to have this problem. My insulin levels were slightly low. Not so low as to cause extremely high blood sugar (although it would occasionally get somewhat high) but low enough to cause poor glucose uptake when my blood sugar was normal. I felt really sluggish and achy because my muscles weren't getting enough glucose.
Metformin kinda worked, but it takes several hours to work, such that if I took metformin in the morning, I wouldn't start to feel better until mid-afternoon. And if my blood sugar was still low then I would have to raise it with hydrocortisone.
Ultimately I needed to fix my pancreas, which I did by taking valproate, a drug which stimulates growth of the pancreatic islets. This also is not a fast process. When I took the depakote, it just messed with my blood sugar and initially made things a bit more unstable. Valproate also is quite toxic to borrelia and caused a herx. After a few weeks of this I got sick of the herxing and quit taking it, and my glucose homeostatis was still crap.
about
a month or two later, my blood sugar suddenly started to get a lot better, and it wasn't just more insulin but also more glucagon which would keep my blood sugar from getting too low.
The basic explanation for this (which has largely been confirmed in animal models) is that it takes 2-3 months for a pancreatic islet cell to fully mature. A few weeks of valproate will start the process, but it takes a couple months for the newly formed cells to transform into the different cell types which produce glucagon (alpha cells) and insulin (beta cells).
If you don't want to take valproate, extreme fasting can also initiate this process, but I found that taking a pill once a day was easier than this extreme lifestyle change. However, people have had success with fasting (and this has also been studied in animals).
People often say they beat diabetes with diet and exercise, but for me that was definitely not happening since I was too sick with Lyme, so instead of diet and exercise, I found that it is totally possible to 'cheat' and do it with drugs.
Surprisingly, the improvement in my blood sugar has lasted for several years now, and it did not just go back to how bad it was before.